The present invention relates to a mechanical pencil with automatic and manual lead advance, and more particularly to a temporary locking mechanism for a sleeve or slider for advancing a thin lead.
A mechanical pencil with automatic lead advance is known having a lead-protecting sleeve which surrounds the portion of the lead projecting from the tip of the pencil. As the lead wears away the sleeve automatically slides back into the body of the pencil. It has been suggested to combine the sleeve or slider structure with an automatic lead-feed mechanism inside the pencil. In this arrangement, the sleeve is spring-biased forwardly toward the writing tip. The lead projecting from the pencil tip is held in a slitted collet chuck. When the tip of the pencil is pressed against the writing paper, the lead is pushed backwardly, frictionally engaging the holder to clamp the lead. When the writing pressure is let up, the arrangement automatically advances the lead a tiny increment.
In the automatic lead-feed mechanical pencil, a rearward pressure applied to a sleeve of the slider or the lead projecting from the sleeve by pressing the projecting lead or a forward end of a slider sleeve against a writing paper forces the slider which holds the lead therein and a collet chuck mechanism into their retracted position. When the rearward pressure is released by lifting the pencil from the writing paper to permit the retracted elements to move into their original forward position, one of the slider and the collet chuck mechanism is temporarily held and the other one of them is first displaced into its forward position to advance the lead. Specifically, by temporarily holding the slider to permit the chuck mechanism to advance before the holding of slider is released, or by holding the chuck mechanism temporarily to permit the slider to advance before the holding of the chuck mechanism is released, an incremental advance of the lead is achieved so that lead projects from the forward end of the slider sleeve.
In such a mechanism, it has been suggested to employ locking engagement between locking jaws or a hook and a groove or grooves formed on the lead gripping holder or collet chuck. In this arrangement, however, stronger pressure should be successively applied to the slider sleeve for the purpose of disengaging the locking engagement, and in that case a forward movement of the collet chuck should be accelerated such that the jaws engaged with the groove can jump over the groove so as to prevent the jaws from reengaging with the groove. Accordingly, a rapid displacement or release of the pencil tip, which has been pressed against the writing paper for lead advancing, from the writing paper as well as a considerable strength of a spring for carrying out the jumping over the groove is required.
In order to improve the aforesaid pencil structure, the inventors of this application have suggested providing an annular elastomeric ring on the outer surface of the slider or the inner surface of a tubular pencil body, and an annular groove on the inner surface of the pencil body or the outer surface of the slider, as described in copending U.S. patent appln. Ser. No. 244,326, filed Mar. 16, 1981 now abandoned. This structure provides a desired temporary holding mechanism for advancing the lead. The inventors have carried out many experiments with the combination between the elastomeric ring and the groove, and have found that there is room for improvement in the sharpness and smoothness of instantaneous disengagement.